Gear differentials used in automobiles include a planetary gear set carried within a differential housing. The planetary gear set interconnects a pair of output drive shafts for opposite directions of rotation with respect to the housing. An input drive shaft is connected to the housing for rotating the housing about a common axis of the output shafts.
In most automotive differentials, the planetary gearing interconnects the output shafts at a speed ratio of minus one. At such a speed ratio, a single rotation of one output shaft with respect to the housing produces a single rotation of the other output shaft in an opposite direction with respect to the housing. In this way, the two output shafts can rotate at different speeds, which average to the rotational speed of the housing.
While rotating at such different speeds (i.e., differentiating), input torque is divided between output shafts in accordance with the efficiency of the planetary gear interconnection. A ratio of the torques between two relatively rotating output shafts is referred to as "bias ratio". The same ratio of torques (or a little larger) is required to initiate differentiation. Bias ratios of two-to-one or more make better use of traction under uneven traction conditions by preventing one wheel from spinning until additional torque distributed to the higher traction wheel reaches the bias ratio.
The planetary gear sets that support bias ratios of two-to-one or more must be of rugged construction to operate at reduced efficiencies. Most friction is generated between the planetary gears and their mounting surfaces. Special care is taken to distribute bearing loads throughout the differentials to avoid producing excessive wear or strain on any one part.
Sun gear members of the planetary gear sets, also referred to as "side" gears, are coupled to inner ends of the two output shafts. Planet gear members of the same sets, also referred to as "element", "spider", or "combination" gears, operatively interconnect the two sun gears for opposite directions of differentiation.
One well-known arrangement of planetary gearing positions side and planet gears within the housing for rotation about axes that extend parallel to each other. Differentials with this type of gearing arrangement are referred to as "parallel-axis" gear differentials. The planet gears of these differentials are generally mounted in pairs. One meshing portion of each planet gear engages one of the side gears, and another meshing portion of each planet gear engages its paired planet gear.
The planet gears arc individually supported for rotation on shafts that project from opposite ends of the planet gears or within pockets formed in the housing. Bores formed in the housing provide bearings for the shafts. Alternatively, the pockets provide bearings for supporting outside cylinder surfaces of the planet gears, including top lands of the planet gear teeth.
An example of a parallel-axis gear differential having planet gears mounted in pockets is disclosed in U.S. Pat. No. 4,677,876. The side gears are axially separated by a space within which the paired planet gears are engaged in mesh. The space between side gears also provides access for fastening ends of the output shafts within the differential with C-clips. A spacer bar holds the output shafts apart after the C-clips are fastened.
A commonly assigned U.S. Pat. No. 5,122,101 discloses an example of a parallel-axis gear differential in which the planet gears are formed by two gear sections joined by a stem. One of the gear sections has a first meshing portion engaged with one of the side gears and a second meshing portion engaged with its paired planet gear. The other gear section has a third meshing portion that is also engaged with its paired planet gear. The two meshes between the paired planet gears straddle the two meshes between the paired planet gears and the side gears. The additional gear meshes between planet gears provide for more balanced loading throughout the differential.
Ordinarily, the side gears are positioned together between the straddled planet gear meshes. However, commonly assigned U.S. Pat. No. 5,292,291 provides for separating the side gears of similar planetary gearing by extending the stem sections of the planet gears. The space between side gears is used for either fastening ends of the output shafts within the differential or connecting a coaxial input shaft to the differential housing.
However, the extended stem sections, which have a smaller diameter than the meshing portions, can weaken the planet gears, especially in torsion. The maximum diameter of the stem sections is equal to the root diameter of the adjacent meshing portions to avoid interference with the side gear teeth. Also, gear blanks with extended stem sections are more susceptible to heat treatment distortions, which produce bending in the gear blanks.